Multi-stage transformation in annealed Ni-rich Ti 49Ni 41Cu 10 shape memory alloy

Abstract

Multi-stage transformation (MST) in 500 °C annealed Ni-rich Ti 49Ni 41Cu 10 shape memory alloy (SMA) is investigated by differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as solution-treated alloy undergoes B2 ↔ B19 ↔ B19′ two-stage transformations. Ti(Ni,Cu) 2 precipitates are formed in 500 °C annealed specimens. Alloy annealed at 500 °C for 6–24 h exhibits MST. This MST is confirmed by DMA tests and is composed of B2 1 ↔ B19 1 ↔ B19′ 1 and B2 2 ↔ B19 2 ↔ B19′ 2 transformations corresponding to the regions near and far from Ti(Ni,Cu) 2 precipitates, respectively. Experimental results show that the more the annealing time, the more the B2 1 ↔ B19 1 ↔ B19′ 1 transformations and finally only B2 1 ↔ B19 1 ↔ B19′ 1 transformations retain with the transformation temperatures close to those of Ti 50Ni 40Cu 10 SMA.